Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage, alternating current, (VAC) signal and provide a high current welding output signal. Around the world, utility power sources (sinusoidal line voltages) may be 200/208 V, 230/240 V, 380/415 V, 460/480 V, 500 V and 575 V. These sources may be either single-phase or three-phase and either 50 or 60 Hz. Welding power sources receive such inputs and produce an approximately 10-75 volt, DC or AC high current welding output.
There are many types of welding power sources that provide power suitable for welding, including inverter-based welding power sources. As used herein, an inverter-type power supply includes at least one stage where DC power is inverted into ac power. There are several well known inverter type power sources that are suitable for welding. These include boost power sources, buck power sources, and boost-buck power sources.
Traditionally, welding power sources were designed for a specific power input. In other words, the power source cannot provide essentially the same output over the various input voltages. More recently, welding power sources have been designed to receive any voltage over a range of voltages, without requiring relinking of the power supply. One prior art welding power supply that can accept a range of input voltages is described in U.S. Pat. No. 5,601,741, issued Feb. 11, 1997 to Thommes, and owned by the assignee of the present invention, and is hereby incorporated by reference.
Many prior art welding power supplies include several stages to process the input power into welding power. Typical stages include an input circuit, a pre-regulator, an invertor and an output circuit that includes an inductor. The input circuit receives the line power, rectifies it, and transmits that power to the pre-regulator. The pre-regulator produces a dc bus suitable for conversion. The dc bus is provided to the invertor of one type or another, which provides the welding output. The output inductor helps provide a stable arc.
The pre-regulator stage typically includes switches used to control the power. The losses in switches can be significant in a welding power supply, particularly when they are hard switched. The power loss in a switch at any time is the voltage across the switch multiplied by the current through the switch. Hard switching turn-on losses occur when a switch turns on, with a resulting increase in current through the switch, and it takes a finite time for the voltage across the switch to drop to zero. Soft switching attempts to avoid turn-on losses by providing an auxiliary or snubber circuit with an inductor in series with the switch that limits the current until the transition to on has been completed, and the voltage across the switch is zero. This is referred to as zero-current transition (ZCT) switching.
Similarly, hard switching turn-off losses also occur when a switch turns off, with a resultant rise in voltage across the switch, and it takes a finite time for the current through the switch to drop to zero. Soft switching attempts to avoid turn-off losses by providing an auxiliary or snubber circuit with a capacitor across the switch that limits the voltage across the switch until the transition to off has been completed, and the current through the switch is zero. This is referred to as zero-voltage transition (ZVT) switching.
There are numerous attempts in the prior art to provide soft-switching power converters or invertors. However, these attempts often either transfer the losses to other switches (or diodes) and/or require expensive additional components such as auxiliary switches and their control circuits. Thus, an effective and economical way of recovering (or avoiding) switching losses in power converters or inverters is desirable. Examples of various attempts at soft switching are described below.
U.S. Pat. No. 5,477,131, issued Dec. 19, 1995 to Gegner discloses a ZVT type commutation. However, a controlled auxiliary switch and a coupled inductor are needed to implement the ZVT. Also, the primary current is discontinuous.
Some prior art designs require discontinuous conduction mode for diode recovery. One such design is found in U.S. Pat. No. 5,414,613. This is undesirable because of excessive high frequency ripple in the power lines.
Gegner also disclosed a ZVS converter that operated in a multi-resonant mode in U.S. Pat. No. 5,343,140. This design produced relatively high and undesirable RMS current and RMS voltage.
Another multi-resonant converter is disclosed in U.S. Pat. No. 4,857,822, issued to Tabisz. This design causes undesirable high voltage stress during ZVS events and undesirable high current stress during ZCS events.
U.S. Pat. No. 5,307,005 also requires an auxiliary switch. Losses occur when the auxiliary switch is turned off. This merely shifts switching losses, rather than eliminating them. Other designs that "shift" losses are shown in U.S. Pat. Nos. 5,418,704 and 5,598,318.
A circuit that requires an auxiliary controlled switch but does not "shift" losses to the auxiliary switch is shown in U.S. Pat. No. 5,313,382. This is an improvement over the prior art that shifted losses, but still requires an expensive controlled switch.
Another design that avoided "loss shifting" is shown in U.S. Pat. No. 5,636,144. However, that design requires a voltage clamp for recovery spikes, and 3 separate inductors. Also, the voltages on the inductors is not well controlled.
A zero-current, resonant boost converter is disclosed in U.S. Pat. No. 5,321,348. However, this design requires relatively complex magnetics and high RMS current in the switches and magnitudes. Also, a high reverse voltage is needed for the boost diodes.
When it is not practical or cost effective to use a true ZCT and ZVT circuit, an approximation may be used. For example, slow voltage/current transitions (SVT and SCT) as used herein, describe transitions where the voltage or current rise is slowed (rather than held to zero), while the switch turns off or on.
A typical prior art welding power supply 100 with a pre-regulator 104 and an output convertor or inverter 105 is shown in FIG. 1. An input line voltage 101 is provided to a rectifier 102 (typically comprised of a diode bridge and at least one capacitor). Pre-regulator 104 is a hard-switched boost converter which includes a switch 106 and an inductor 107. A diode 108 allows a capacitor 109 to charge up by current flowing in inductor 107 when the switch 106 is turned off. The current waveform in inductor 107 is a rectified sinusoid with high frequency modulation (ripple).
The amount of ripple may be reduced by increasing the frequency at which switch 106 is switched. However, as the frequency at which a prior art hard switched boost converter is switched is increased to reduce ripple, the switching losses can become intolerable.
Another drawback of some prior art power supplies is a poor power factor. Generally, a greater power factor allows a greater power output for a given current input. Also, it is generally necessary to have more power output to weld with stick electrodes having greater diameters. Thus, a power factor correction circuit will allow a given welding power supply to be used with greater diameter sticks for a given line power. A prior art inverter that provided a good power factor is disclosed in U.S. Pat. No. 5,563,777. Many prior art convertors with power factor correction suffer from high switching losses. Examples of such prior art designs are found in U.S. Pat. Nos. 5,673,184; 5,615,101; and 5,654,880.
One type of known output convertor is a half-bridge, transformer isolated, inverter. However, such output invertors often have high switching losses and/or require passive snubber circuits (which increases losses) because each snubber must operate in both directions overall, but only in one direction at a time. Also, known snubber circuits generally have a limited range of acceptable loads and will not snub proportional to the load, thus the losses are relatively high for lower loads.
Accordingly, a power circuit that provides little switching losses and a high (close to unity) power factor is desirable. Also, the pre-regulator should be able to receive a wide range of input voltages without requiring relinking. A desirable output convertor will include a full wave, transformer isolated, inverter, that is soft switch and has full range, full wave, low loss snubber.